The invention is concerned with an electrochemical gas analyzer for determining the sulphur dioxide content of certain gases, in particular, flue gases. The device comprises a measuring cell containing a measuring electrode for the determination of the depolarization current and further comprising an unpolarizable electrode in the same electrolyte. The gas input flow is utilized to cause a circulation of electrolyte containing the dissolved gas which moves in the space between the measuring and counter-electrode. The electrolyte is kept in circulation and constantly renewed. The sulphur dioxide dissolved in the electrolyte is removed outside the cell by treatment with air in the presence of activated charcoal.
The control of sulphur dioxide content in flue and other exhaust gases is a particularly important task in the field of measurement technology. All of the previously known available measuring devices which are based on different principles (such as infrared, conductivity, UV, etc.) are either very expensive or can only be used in limited environments.
There has previously been developed an electro-chemical gas analyzer utilizing the foregoing general principles which is disclosed in U.S. Pat. No. 4,409,069. This device is very economical and may be utilized over a substantial range of concentrations. In this analyzer electrolyte is added dropwise to the measuring cell and the used electrolyte removed therefrom. Similarly, the sulphur dioxide is removed from the electrolyte by treatment with air in the presence of activated charcoal outside the cell and recirculated to the cell. Dilute copper sulfate may be utilized therein as the electrolyte.
Since the measuring effect is dependent upon the sulphur dioxide concentration in the electrolyte which increases cumulatively with contact time up to the achievement of the equilibrium value, it is clear that the sensitivity of such a device rises with the increase in saturation time of the electrolyte. For this reason, the change in electrolyte content of the cell has heretofore been provided to be of the order of 0.01 to 0.1 of the total volume per minute (see for example, DE-AS No. 1091776). Such an electrolyte exchange rate is so small that considerations relating to collection, regeneration, and recycling need not usually cause concern and must be dealt with only when substantial concentrations of copper salts are in the electrolyte. Under these circumstances the required removal of sulphur dioxide by oxidation with oxygen in the presence of activated charcoal is achieved by suspension of activated charcoal in electrolyte and the passage of air thru such a suspension.
In practice however, it has been found that the stability and reproduceability of the measurement results and also its only approximate linear relationship to the SO.sub.2 content over a wide measuring range leaves something to be desired. The suggestion has been made in the aforementioned DE-AS that this problem could be solved by efficient temperature control. Regrettably, even the use of a second cell utilized as a reference cell carrying sulphur dioxide free carrier gas as previously conceived by the Applicants, does not obviate the problem.